1. Field of the Invention
This invention relates to a novel and useful preparation of B-trichloroborazine, a compound useful, for instance, in the formation of specialty inorganic polymers. More specifically, the present invention relates to the preparation of B-trichloroborazine at elevated temperatures using ammonia and boron trichloride in a suitable solvent.
2. Description of the Prior Art
B-trichloroborazine (B.sub.3 N.sub.3 Cl.sub.3 H.sub.3, also known as B-trichloroborazole) has long been recognized as a desirable reagent for the preparation of a large number of borazine derivatives, including borazine (borazole) itself, and due to its polyfunctionality it is also considered to be a most excellent starting material for the synthesis of high stability inorganic polymers. However, in spite of the many desirable properties of B-trichloroborazine, its straightforward preparation from boron trichloride and ammonia has not been disclosed.
The preparation of B-trichloroborazine by a number of routes has been reported. These include, for example, P. A. Tierney, who disclosed in the Abstracts of the 141st Annual Meeting of the American Chemical Society held in Washington, D.C., Mar. 20-29, 1962, [Abstract 13, Division of Inorganic Chemistry] that the reaction of boron trichloride and ammonia-produced ammonium chloride, NH.sub.4 Cl, and dichloroboronamide, Cl.sub.2 BNH.sub.2, which on heating decomposed to produce hydrogen chloride, HCl and BN. No evidence for the formation of B-trichloroborazine was obtained in any reactions involving less boron trichloride than is in BCl.sub.3.2NH.sub.3. Mixtures of BCl.sub.3.2NH.sub.3 and BCl.sub.3 were reported as forming B-trichloroborazine under the same conditions generally used for its synthesis from boron trichloride and ammonium chloride. However, these synthesis methods were not described in detail in any later publication of Tierney or others.
G. L. Brennan et al., in the Journal of the American Chemical Society, Vol. 82, p. 6248, published in 1950, disclose the preparation of B-trichloroborazine by passing boron trichloride through a column of ammonium chloride at 200.degree. C., which produced a 40 percent yield.
E. F. Rothgery et al., in Inorganic Chemistry, Vol. 6, No. 5, p. 1065, published in 1967, disclose the preparation of B-trichloroborazine by the combination of boron trichloride and methyl cyanide in chlorobenzene. Ammonium chloride is added and the mixture is refluxed for 2.5 hr to give soluble products. The solvent is removed by vacuum distillation, and the pasty residue was sublimed to give B-trichloroborazine.
D. F. Gaines and Jorgan Borlin in "borazines" (Chapter 7), as found in Boron Hydride Chemistry by Earl L. Muetterties (Ed.), Academic Press, New York, NY, published in 1975, disclose the preparation of B-trichloroborazine using ammonium chloride and boron trichloride. This reference discloses, without experimental details, that yields of B-trichloroborazine from ammonia and boron trichloride are very low, but 50 percent yields of Cl.sub.3 B.sub.3 N.sub.3 H.sub.3 (B-trichloroborazine) are obtained using ammonium chloride rather than ammonia itself.
In the Journal of the American Chemical Society, Vol. 76, 3303 (1954), R. Schaeffer et al. disclose the preparation of B-trichloroborazine using ammonium chloride dissolved in chlorobenzene with specially prepared ground glass heated to 140.degree.-150.degree. C. Boron trichloride is added in excess slowly over a five-hour period. Then excess boron trichloride is allowed to distill off. The reaction mixture is cooled and the supernatant liquid is removed and centrifuged to collect the solids. The centrifugate is separated by fractional distillation to produce a liquid and crude solid B-trichloroborazine which is purified by vacuum sublimation at about 50.degree.-60.degree. C. initially, about a 40 percent yield of B-trichloroborazine is produced. After two or three repetitions of adding additional boron trichloride and ammonium chloride, yields approach 90 percent. Tribromoborazine was prepared in an analogous method to the one used for the chloro-derivative. The reference also discloses that the yields in both steps of the method, i.e., the preparation of trichloroborazole (trichloroborazine) and its reduction to borazole are greatly dependent upon the techniques and conditions employed.
C. A. Brown et al., in "B-Trichloroborazole" (B-trichloroborazine) in the Journal of the American Chemical Society, Vol. 77, pp. 3699-3700, published in 1955, disclose the preparation of B-trichloroborazine using boron trichloride and ammonium chloride at temperatures above 110.degree. C. Two methods of production are described. In the first method, dry powdered ammonium chloride is placed in a long Pyrex combustion tube and heated to 165.degree.-175.degree. C. Boron trichloride in dry nitrogen is passed over the ammonium chloride and B-trichloroborazine condenses as crystals. Yields of about 35 percent based on the amount of unreacted ammonium chloride. In the second method, B-trichloroborazine is prepared by refluxing a mixture of chlorobenzene and ammonium chloride in the presence of boron trichloride vapor. Anhydrous ammonium chloride and anhydrous chlorobenzene are combined and heated to reflex temperature (130.degree. C.). Boron trichloride in dry nitrogen is added over a ten hour period. The solvent is removed by filtration in the absence of moisture. The crude solid residue is purified by vacuum sublimation. A 36 percent yield of B-trichloroborazine is obtained based on unreacted ammonium chloride. The reference also discloses that little, if any, reaction of boron trichloride and ammonium chloride occurs below 110.degree. C. Above this temperature, hydrogen chloride, B-trichloroborazine and non-volatile products are formed. Further, attempts to prepare B-trichloroborazine by the reaction of boron trichloride with ammonia gas resulted in only traces of the desired compound. In this case, the products were almost wholly white, non-volatile solids.
A United Kingdom No. 883,327, assigned to the United States Borax & Chemical Corporation, discloses a method of producing B-trichloroborazole which includes fluidizing a bed of ammonium bromide, fluoride, sulfate or orthophosphate particles by means of a substantially moisture-free gas and heating the bed until it is substantially completely moisture-free, charging the heated fluidized bed with substantially completely moisture-free boron trichloride gas, and condensing the B-trichloroborazine resulting from the reaction of the boron trichloride and the reactants cited above. The temperatures of the reaction are 120.degree. C. to 350.degree. C., and yields are of the order 70-80 percent. Also, see U.S. Pat. No. 3,030,174 for a similar process.
U.S. Pat. Nos. 3,394,999; 3,321,337; 3,261,378; 3,180,701; 3,018,307; and 2,996,491 disclose either the preparation, use or subsequent reaction of B-trichloroborazine, and are cited of general interest only. Finally, an overview of the preparation of boron-nitrogen compounds is found in "Organic Boron Nitrogen Compounds" by H. D. Smith, Jr. in Kirk-Othmer: Encyclopedia of Chemical Technology, Vol. 4, 3rd ed., pp. 188-201, published in 1978.
None of the above references disclose the present invention. As can be seen from the above-cited art, the attempted preparation of B-trichloroborazine from boron trichloride and ammonia has produced no reaction or only trace quantities of the expected product. It is therefore extremely desirable to have an easy and convenient preparation of B-trichloroborazine from boron trichloride and ammonia at ambient pressure and easily obtained temperatures.